The present invention is directed, in general, to a storage device and, more specifically, to a system and method of holographic storage having an enhanced temperature operating range.
The technologies supporting the development of enhanced information systems is a major area of focus. Optical storage of data has been one of the bright spots in these technologies over the past several years. For example, compact discs dominate the market for musical recordings and are now also the standard medium for multimedia releases, which may combine text, images and sound. A compact disc can hold about 640 megabytes that can accommodate 300,000 pages of doubled-spaced typewritten text or one and a quarter hour of high-fidelity music. However, developers of information storage devices continue to seek increased storage capacity.
As part of this development, page-wise memory systems employing holographic storage have been suggested as an alternative to conventional memory devices. Page-wise memory systems that store a page of data involve the storage and readout of an entire two-dimensional representation. Typically, recording or xe2x80x9cwritingxe2x80x9d light passes through a two-dimensional array of dark and transparent areas representing data. Then, the holographic system stores the data in three dimensions where the holographic representations of the pages occur as patterns of varying refractive index imprinted into a storage medium. The holographic data storage typically consists of a distribution of gratings having varying tilt angles with separations proportional to periods caused by the angular bandwidth of the data pages that are recorded. Additionally, reconstructive or xe2x80x9creadingxe2x80x9d light is diffracted at a well-defined angle of incidence (the Bragg angle) with respect to the gratings. Background information concerning holographic systems is discussed in Holographic Memories, by D. Psaltis, et al., Scientific American, November (1995) and incorporated herein by reference.
Photopolymer materials are considered attractive recording media candidates for high density holographic data storage. They are low in cost, are easily processed and can be designed to have large index contrasts with high photosensitivity. This class of materials can be fabricated with the dynamic range, media thickness, optical quality and dimensional stability required for high density applications. This are discussed in xe2x80x9cRecordinq Media That Exhibit High Dynamic Range for Holographic Storagexe2x80x9d, by Lisa Dhar et. al., Optics Letters, Volume 24, P.487 (1999) and incorporated herein by reference. An area of disadvantage for these materials is their fairly large coefficients of thermal expansion, which produces a dimensional change in the material-with changes in temperature.
Polymer materials for holographic recording are typically sandwiched between two substrates to insure high optical quality. Currently, glass substrates are used to sandwich the polymer material. The dimensional changes caused by temperature variations of the polymer in glass substrates exhibit anisotropic behavior in that the variations occur mainly in the thickness (perpendicular to the plane of the substrates) direction. This occurs because the polymer material is constrained by the rigid substrates in the lateral (parallel to the plane of the substrates) direction and only allowed to displace in the thickness direction. This anisotropic temperature response produces a negative effect on the fidelity of data recovery that is about three times greater than the effect of an isotropic temperature response. This behavior seriously restricts the acceptable operating temperature range of the material""s use as a holographic data storage medium. Background information concerning anisotropic temperature effects are discussed in xe2x80x9cTemperature-Induced Changes in Photopolymer Volume Hologramsxe2x80x9d, by Lisa Dhar, et al.,Applied Physics Letter, Volume 73 No. 10, 1337 (1998) and incorporated herein by reference.
Accordingly, what is needed in the art is a way to diminish the effects of temperature variation on stored holographic information.
To address the above-discussed deficiencies of the prior art, the present invention provides a holographic storage medium, a method of manufacturing the holographic storage medium and a holographic storage device incorporating the storage medium. In one embodiment, the holographic storage medium includes: (1) first and second spaced-apart substrates, the first substrate being plastic and (2) a photopolymer core located between the first and second substrates and having a coefficient of thermal expansion such that the first and second substrates and the photopolymer core cooperate to respond substantially isotropically to a change in temperature.
The present invention recognizes that dimensional stability in at least one of the substrates of a holographic storage medium is not necessarily desirable; rather, mechanical compatibility between the core and the at least one substrate is more important in that material expansion can become isotropic and therefore of less optical effect. This results in a broader operating temperature range for the medium and greater writing and reading reliability.
For purposes of the present invention, the term xe2x80x9crespond substantially isotropicallyxe2x80x9d means that the expansion does not exceed the operating limits for Bragg Angle Shifts for the material used to construct the holographic storage medium. One skilled in the pertinent art is familiar with Bragg Angle Shifts. Alternatively, although the response may not be substantially isotropic, the degree of anisotropicity is such that an enhanced operating temperature range is maintained without incurring unacceptable writing or reading errors. The effects described above are noted for single holograms as well as holograms that are multiplexed in a volume using any current or future developed multiplexing techniques. For example, angle multiplexing, wavelength multiplexing, phase correlation multiplexing, aperture multiplexing, shift multiplexing and phase code multiplexing.
Additionally, photopolymer cores may include nonphotoactive materials as well as photoactive monomer systems. Photoactive monomer systems are systems that include monomers that will polymerize on the incidence of light. Photoactive monomer systems are used in photopolymers that use polymerization as a mechanism of recording and are well known to one skilled in the pertinent art.
In one embodiment of the present invention, the coefficient of thermal expansion of the photopolymer core ranges from about 50% to about 500% of a coefficient of thermal expansion of the first substrate. In a related embodiment, the coefficient of thermal expansion of the photopolymer core further ranges from about 50% to about 500% of a coefficient of thermal expansion of the second substrate.
In one embodiment of the present invention, the second substrate is plastic. In an embodiment to be illustrated and described, the second substrate comprises the same material and is of the same lateral dimension as the first substrate, although this certainly need not be the case. In other embodiments of the present invention, the materials making up the first and second substrates may be different or the same.
In one embodiment of the present invention, optical effects of the coefficient of thermal expansion of the photopolymer core are wavelength-depended. In another embodiment of the present invention, optical effects of the coefficient of thermal expansion of the photopolymer core can be compensated by tuning the wavelength of the readout laser. This allows a tunable laser to be employed further to compensate for optical variations caused by thermal expansion or contraction.
In one embodiment of the present invention, a fraction of photopolymer core is a photoactive monomer system. In another embodiment of the present invention, the entire photopolymer core is a photoactive monomer system.
In one embodiment of the present invention, exposure to light polymerizes the photopolymer core or a fraction of the photopolymer core. However, other chemical or structural changes within the core signifying the presence, degree or absence of an interaction are within the broad scope of the present invention.
In one embodiment of the present invention, the present invention provides a holographic storage device that includes: (1) coherent light source, (2) a holographic multiplexing mechanism that causes changes in an interaction between an object beam and a reference beam derived from said coherent light source and (3) a holographic storage medium that receives and stores interference patterns resulting from said interaction and including: (A) first and second spaced-apart substrates, said first and second substrates being plastic, and (B) a photopolymer core located between said first and second substrates and having a coefficient of thermal expansion such that said first and second substrates and said photopolymer core cooperate to respond substantially isotropically to a change in temperature.
For purposes of the present invention, the term xe2x80x9ccauses changes in an interactionxe2x80x9d means that the holographic multiplexing mechanism can cause a change in the period or phase of grading in the interaction. Of course, however, other types of changes to the interaction are well within the broad scope of the present invention.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.